Problem: The ratio of measures of two complementary angles is 4 to 5. The smallest measure is increased by $10\%$. By what percent must the larger measure be decreased so that the two angles remain complementary?
If the ratio of the two complementary angles is 4 to 5, then there are 9 equal parts making up the full 90 degrees. That means each part is 10 degrees, and the two angles are 40 degrees and 50 degrees. When the 40-degree angle is increased by $10\%$, we get 44 degrees. The 50-degree angle must drop down to 46 so that the two angles remain complementary. Dividing 46 by 50, we get 0.92, or $92\%$. The larger angle must decrease by $\boxed{8\%}$.